1. Field of the Invention
The present invention relates to a system allowing selective connection of various types of external apparatuses such as a handset and a headset to a telephone set.
2. Description of the Related Art
Referring to FIG. 6, there is shown a block diagram of a prior art key telephone set system in which a voice signal, which is transmitted through a voice signal transmission line 1 from a main unit 100 connected to outside lines not shown, is applied to a hybrid circuit 3 through a voice signal transformer 2 and further from the circuit 3 to a receiver 9 of a handset 8 through a receiving-voice amplifier 6 of a talking circuit LSI 5 and a connecting terminal 7a. A voice signal emitted from a transmitter 10 of the handset 8 is applied to the hybrid circuit 3 through a connecting terminal 7b, a transmitter interface circuit 11 and a transmitting-voice amplifier 12 of the talking circuit LSI 5, and further from the hybrid circuit 3 through a voice transformer 2 to the voice-signal transmission line 1 leading to the main unit 100.
On the other hand, a control signal, which is transmitted through a data transmission line 13 from the main unit 100, is applied through a data transformer 14 to a CPU 15 which in turn controls the talking circuit LSI 5 and a light emitting diode 16 in response to the received control signal. When a dialing key 17, a button switch 18 or a hook switch 19 is operated, the CPU 15 sends a control signal indicative of the operated switch or key through the data transformer 14 to a transmission line 13 leading to the main unit 100.
Recent demand for improving the reliability of the system and lowering the power consumption, cost and 1 he like thereof has led to the formation of the hybrid circuit and transmitter interface circuit in a semiconductor form and the employment of a transmitter of a type other than a carbon type, e.g., an electret type, a dynamic type or the like. However, the electret and dynamic type transmitters require a transmitter interface circuit different in type from that in the case of the carbon type transmitter, because the electret and dynamic types have characteristics such as output impedance, output signal level and operating DC current which characteristics are much different from those of the carbon type. For this reason, even when it is desired from a design demand to connect the dynamic type transmitter including the transmitter interface circuit with a handset having a carbon type transmitter or with various external apparatuses (including, for example, a headset, an external transmitting speech amplifier, a modem and so on) having interface conditions meeting the carbon type transmitter, such interconnection has been impossible because of their mismatching of the aforementioned output impedance, output signal level, DC current and the like.
In this way, the prior art telephone set system has had such a problem that, for example, in the case where a transmitter interface circuit is incorporated in a dynamic type transmitter, even if it is desired to connect to the dynamic type transmitter a handset having a carbon type transmitter or various types of interface units having interface conditions meeting the carbon type transmitter, such interconnection is impossible because of their mismatching of the output impedance, output signal level DC current and the like.